The invention relates to a method of determining structural features, in particular structural defects, of test objects having a randomly scattering surface.
It is known that streak-projection methods, such as the Moirxc3xa9 Technique and interferometric methods such as the ESPI (electronic speckle pattern interferometry) or the shearing technique can be used to determine the areas of displacement or expansion of a test object having a randomly scattering surface. The desired result and the required resolution or rigidity of the test object relative to the magnitude of the applied forces determine which method can be used with a given test object. For the above-mentioned test methods, two states of the test object are normally compared during a static test in that the object is recorded at two different states of stress and the interferograms for the two states are subtracted. Depending on the measuring principle used, the resulting difference interferogram represents either the displacement or the expansion of the object between the two states in the form of interference lines. The amount of the displacement or expansion at one picture point in the difference interferogram can then be determined, for example, through counting the interference lines, starting with a picture point with known displacement or expansion and by taking into account the wave length for the light that is used.
If the sensing head is provided with a phase shifting unit, an expanded evaluation based on the principle of the phase shifting method can be carried out (W. Osten, xe2x80x9cDIGITALE VERARBEITUNG UND AUSWERTUNG VON INTERFERENZ-BILDERNxe2x80x9d [Digital Processing And Evaluation Of Interference Patterns], Chapter 6, Publishing House Akademie Verlag ISBN 3-05-501294-1). In the process, phase pictures are created, which assign a specific phase angle to each picture point. If the phase pictures of two object states are subtracted, a phase difference picture is obtained. In contrast to the above-mentioned difference interferogram, the phase difference picture does not show sinusoidal modulated interference lines, but shows directly the phase angle between the second and the first state. A further advantage of this representation is that the phase angle is standardized, owing the computing rule used for the phase shifting method. That is to say, the gray value that corresponds in a phase picture with a phase angle is always constant, independent of the picture coordinate. The disadvantage of the phase shifting method is that during the feeding in of the picture sequence that is required for the phase shifting method, the test object must be absolutely still. To avoid this disadvantage, a test method has been developed (see German Patent Specification 3843396 C1), which is known under the designation xe2x80x9cdirect phase measurementxe2x80x9d or xe2x80x9cspatial phase shifting method.xe2x80x9d This method only requires a grid projection or a camera picture for computing 2xcfx80 modulated phase pictures.
To make an evaluation of the phase difference pictures easier for the examining person, the pictures should advantageously be processed in such a way that the defects are clearly visible to said person. For this, it is particularly useful to eliminate the frequently occurring whole-body movements or other non-relevant global object deformations, which are superimposed on the local deformations in order to facilitate the detection of local structural features, particularly structural defects.
German Patent 19501073 A1 xe2x80x9cBILDVERARBEITUNGSVERFAHREN ZUR ERMITTLUNG DER STRUKTURFESTIGKEIT EINES PRxc3x9cFOBJEKTS MIT DIFFUS STREUENDER OBERFLxc3x84CHExe2x80x9d [Picture Processing Method For Determining The Structural Stability Of A Test Object Having A Randomly Scattering Surface] describes an evaluation method for phase pictures used in shearography and designed to process local defects so that they are clearly visible to the examining person. For this, the phase difference picture is initially stabilized, is displaced in the picture processing system by a fixed amount xcex94X, and the displaced picture and the non-displaced picture are subtracted from each other. In the most favorable case, the error is thus displayed with dual amplitude. However, this method has considerable disadvantages. A demodulation or stabilization of the phase-difference picture is necessary. For the demodulation, the initially unknown 2xcfx80 offset of the determined 2xcfx80 modulated phase angle is reconstructed for each picture point in that the environment is examined for specific continuity characteristics. However, this procedure is subject to errors, wherein a demodulation error that occurs only once will propagate throughout its environment during a further demodulation. Error values that differ considerably from xcex94X furthermore have exactly the opposite result of that which is intended because the amplitude distribution within the error is smaller than before.
Starting with this state of the technology, it is the object of the invention to specify a method for determining structural features of test objects having a randomly scattering surface, which method generates a result picture, in which the originally existing global deformations are removed. As a result, the examining person can easily detect locally limited structural features, particularly structural defects, in surveyed test objects.
The above object is achieved with the method according to the invention described below wherein advantageous modifications are specified and described.
According to the invention, the surface of the object to be examined is illuminated with coherent or structured light and is monitored with a camera, which is preferably a CCD video camera. The test object is subjected to different stresses to examine its structural characteristics. These stresses can include pressure, tension, bending or other types of stresses. The displacements or expansion of the object surface, caused by the various stresses, are recorded on the camera picture sensor as intensity modulation in the images of the object to be surveyed, caused by interferometry or structured light (e.g. streak pattern). With the speckle measuring techniques, for example, the deformation of the object surface is recorded as sinusoidal intensity modulation (sinusoidal intensity curve) of the speckles near the object that are imaged on the picture sensor. The images generated on the picture sensor of the monitoring camera are supplied to a picture processing system for further processing. Preferably, this is a digital picture processing system. A phase difference picture that shows the displacement or the stress/expansion state of the object is generated from the images that are produced and stored in the picture processing system. The phase difference picture shows for each point the difference between the phase angle of the light impinging on the picture sensor for one state of stress and the phase angle of the light impinging on the picture sensor for a different state of stress. A phase shifting process can be used, for example, to generate the phase difference picture.
According to the invention, a copy of the phase difference picture is generated, which is manipulated relative to the original phase difference picture in such a way that the structural features are removed or at least strongly suppressed. The manipulated copy of the phase difference picture is subsequently linked to the original phase difference picture. By using a suitable link, preferably the subtraction of the manipulated copy of the phase difference picture from the original phase difference picture, a phase difference picture results for which global deformations, whole-body movements or setting operations are eliminated. In contrast, the structural features eliminated in the manipulated phase difference picture continue to exist completely or nearly completely in the resulting phase difference picture.
In accordance with one advantageous modification of the invention, a low-pass filter is used to remove the structural features in the manipulated phase difference picture, since significant structural features, particularly structural defects, typically appear as local extreme values with little expansion of area in the phase difference picture. In order to manipulate the phase picture by means of a low-pass filter, this picture is preferably first divided into a sine picture and a cosine picture. For this, the sine value and the cosine value are computed point-by-point from the phase value and are filed as a sine picture and a cosine picture. As a result of this transformation, two sinusoidal modulated streak pictures are obtained from the phase picture. In contrast to the phase picture, these streak pictures do not show any locations of unsteadiness in the form of phase jumps and are thus suitable for a low-pass filtering. Following the low-pass filtering, the phase picture can be computed anew from the sine picture and the cosine picture by using the function of arc cotangent.
In order to eliminate the structural features in the manipulated phase difference picture, the low-pass must be selected strong enough so that the largest features to be expected are for the most part eliminated. If a filtering matrix is used according to one embodiment of the invention, then the filtering matrix is selected to be approximately as large as the largest expected structural feature. With a standard camera resolution, for example according to the CCIR Standard with 768xc3x97576 pixels, and a maximum size for the structural features of 10% of the picture size, the size of the filtering matrix is thus approximately 76xc3x9776. The filtering with such a large filtering matrix requires a considerable amount of time, even with state of the art computer systems, especially since the filtering takes place on the sine picture as well as the cosine picture. In accordance with another embodiment of the invention, a recursive low-pass filter can alternately be used. The advantage of this filter is that regardless of the selected filter strength, the filtered value of a picture point can be computed respectively only from the picture point in the preceding column and the preceding line, as seen in filtering direction. In contrast to most other filters, the filtered value is re-recorded into the picture with the recursive low-pass and is used again for the following picture point to be filtered (thus the term xe2x80x9crecursivexe2x80x9d). The linking to the picture point of the preceding column or the preceding line occurs based on the following formulas:
Linking to the preceding column:
Ixe2x80x2(x,y)=(1xe2x88x92k)xc3x97I(xxc3x971, y)+kxc3x97I(x,y)
Linking to the preceding line:
Ixe2x80x2(x,y)=(1xe2x88x92k)xc3x97I(x,yxe2x88x921)+kxc3x97I(x,y)
With:
x: column coordinate for the picture point
y: line coordinate for the picture point
I(x, y): original intensity value at the picture point (x, y)
Ixe2x80x2(x,y): filtered intensity value at the picture point (x, y)
I(xxe2x88x921,y): intensity value of the picture point in the preceding column in filtering direction
I(x,yxe2x88x921): intensity value of the picture point in the preceding line in filtering direction
k: filtering intensity (real number at interval ]0 . . . 1[)
The picture is increasingly distorted if the recursive low-pass filter is adjusted to be extremely strong (that is to say k is near 0), meaning the picture is also geometrically distorted in addition to the desired low-pass effect. In order to minimize these effects and according to one embodiment of this filtering method, the filter is used repeatedly with a lower intensity (meaning a higher k value). In the process, the filtering is started from different corner points of the picture to be filtered, so that the geometric distortions are for the most part compensated.
One exemplary embodiment of the invention is explained in the following with the aid of drawings.